This invention relates to a radio loudspeaker telephone device for use in a mobile radio telephone system comprising a radio channel.
The radio loudspeaker telephone device is for carrying out communication of a speech signal with a subscriber telephone set through a radio base station and a central exchange of a central station without using a handset of the radio loudspeaker telephone device. For this purpose, the radio loudspeaker telephone device comprises a microphone for producing a microphone output speech signal and a loudspeaker for responding to a loudspeaker input speech signal.
The radio loudspeaker telephone device further comprises a hook switch, a dialing unit, a radio transmitter, and a radio receiver. When an operator of the radio loudspeaker telephone device makes the device execute calling operation, the operator puts the hook switch in an off-hook state at first to make the hook switch produce an off-hook state signal indicating that the device is put in the off-hook state. Thereafter, the operator makes the dialing unit generate a dial signal representative of a destination subscriber telephone set.
A radio controller is connected to the hook switch and the dialing unit. The radio controller controls the radio transmitter and the radio receiver in response to the off-hook state signal so that the radio transmitter transmits the dial signal to the radio channel and that the radio receiver receives a response signal from the radio channel and delivers the response signal to the radio controller. The response signal indicates that the destination subscriber telephone set is not busy.
The radio controller controls the radio transmitter and the radio receiver in response to the response signal so that the radio transmitter transmits the microphone output speech signal to the radio channel and that the radio receiver receives a ring-back tone signal from the radio channel and delivers the ring-back tone signal to the loudspeaker as the loudspeaker input speech signal. The ring-back tone signal indicates that the destination subscriber telephone set is called by the device with the destination subscriber telephone set kept in an on-hook state.
When the destination subscriber telephone set is put in an off-hook state, a loop is terminated or established which is constituted by the destination subscriber telephone set, the loudspeaker, and the microphone. However, the loop is unterminated or unestablished so far as the destination subscriber telephone set is kept in the on-hook state. Unless the loop is terminated, howling or singing inevitably occurs due to acoustical coupling between the loudspeaker and the microphone.
As will later be described, a conventional radio loudspeaker telephone device is incapable of automatically preventing the howling or the singing on execution of the calling operation.